What is Dichloroiodomethane and Why Does It Matter?

Dichloroiodomethane is a disinfection byproduct — a chemical that forms when water treatment plants use disinfectants like chloramine or chlorine. It wasn't added to your water on purpose. It's created when those disinfectants react with naturally occurring organic matter, like decaying leaves or algae, along with iodine already present in the source water. The result is a halogenated compound (a chemical containing iodine and chlorine) that ends up in your tap water as an unintended side effect of the treatment process meant to keep you safe from bacteria.

The health research on dichloroiodomethane is still developing, but early findings raise legitimate concerns. Studies suggest it may be genotoxic, meaning it could damage DNA at the cellular level. The Environmental Working Group (EWG) classifies it as a possible human carcinogen based on available toxicological data. Like many disinfection byproducts, the risk grows with long-term exposure — drinking small amounts daily over years is more concerning than a single exposure. Children, pregnant women, and people with compromised immune systems face the greatest potential risk from ongoing exposure to these types of compounds.

There is currently no federal Maximum Contaminant Level (MCL) set by the EPA specifically for dichloroiodomethane. It falls into a category of emerging contaminants that regulators are still working to evaluate. The EWG has established a health guideline, and while none of the 15 water systems where dichloroiodomethane was detected exceeded that guideline, detection itself matters. The average level found across those systems was 0.831 parts per billion (ppb), with a maximum detected level of 2.0 ppb. The absence of a legal limit doesn't mean the absence of risk — it often just means regulation hasn't caught up with the science yet.

Geographically, this contaminant shows a clear concentration in Texas, where 14 of the 15 affected water systems are located. One system in New York also detected it. Texas's pattern makes sense when you consider the state's water sources. Many Texas utilities draw from surface water — rivers and reservoirs — that carry high levels of organic matter, especially during warm months. Add iodine-rich groundwater blending and the widespread use of chloramine disinfection, and the conditions are right for dichloroiodomethane to form. Warmer water temperatures also accelerate the chemical reactions that produce disinfection byproducts, which is why southern states tend to see higher levels than cooler northern regions.

The good news is that effective filtration options exist. Activated carbon filters can reduce dichloroiodomethane, but the most reliable protection comes from a reverse osmosis (RO) system. Reverse osmosis pushes water through a semipermeable membrane that blocks a wide range of contaminants, including disinfection byproducts like this one. An Echo Water reverse osmosis system is designed specifically to address contaminants that standard pitcher filters or faucet attachments can miss. If you're in Texas or another area where disinfection byproducts are common, point-of-use filtration at your kitchen tap is one of the most practical steps you can take. It won't change what comes into your home, but it will change what ends up in your glass.

Regulatory Standards for Dichloroiodomethane

Cities With the Highest Dichloroiodomethane Levels

States Most Affected by Dichloroiodomethane

Frequently Asked Questions

Related Contaminant Guides